This invention relates to the control of power levels of transmitted signals in telecommunication systems, in particular spread spectrum systems.
Good transmit power control methods can be important to communication systems having many simultaneous transmitters because such methods reduce the mutual interference of such transmitters. For example, transmit power control is necessary to obtain high system capacity in interference limited communication systems, e.g., those that use code division multiple access (CDMA). Depending upon the system characteristics, power control in such systems can be important for the uplink (i.e., for transmissions from a remote terminal to the network), the downlink, (i.e., for transmissions from the network to the remote terminal) or both.
In a typical CDMA system, an information data stream to be transmitted is superimposed on a much-higher-bit-rate data stream produced by a pseudorandom code generator. The information signal and the pseudorandom signal are typically combined by multiplication in a process sometimes called coding or spreading the information signal. Each information signal is allocated a unique spreading code. A plurality of coded information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, the corresponding information signal can be isolated and decoded.
The need for transmit power control in the uplink is recognized in current CDMA cellular systems, as may be seen from "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", TIA/EIA Interim Standard TIA/EIA/IS-95 (Jul. 1993) and its revision TIA/EIA Interim Standard TIA/EIA/IS-95-A (May 1995). Such standards that determine the features of U.S. cellular communication systems are promulgated by the Telecommunications Industry Association and the Electronic Industries Association located in Arlington, Va.
Uplink power control according to the IS-95-A standard is provided by, among other techniques, a closed-loop method in which a base station measures the strength of a signal received from a remote station (e.g., relative to its associated noise) and then transmits one power control bit to the remote station every 1.25 milliseconds. Based on the power control bit, the remote station increases or decreases its transmit (uplink) power by a predetermined amount. According to Sections 6.1.2.3.2 and 7.1.3.1.7 of the standard, a "zero" power control bit causes the remote station to increase its transmit power level by 1 dB and a "one" power control bit causes the remote station to decrease its transmit power level by 1 dB. The IS-95-A standard also addresses uplink power control in other situations, such as when a remote station accesses the system (before the closed-loop power control method is active), using an open loop power control technique wherein the remote station gradually increases its transmit power level until the network responds to its access attempts.
Similar concerns exist in the downlink. To achieve reliable reception of a signal at each remote station, the ratio of the signal to the interference (SIR) should be above a prescribed threshold for each remote station (referred to as a "required signal-to-interference" level, or SIR.sub.req). For example, as shown in FIG. 1, consider the case where three remote stations receive, respectively, three signals from the common CDMA communication band. Each of the signals has a corresponding energy associated therewith--namely energy levels E1, E2 and E3 , respectively. Also, present on the communication band is a certain level of noise (N). For the first remote station to properly receive its intended signal, the ratio between E1 and the aggregate levels of E2, E3 and N must be above the first remote station's required signal-to-interference ratio.
To improve the SIR for a remote station, the energy of the signal is increased to appropriate levels. However, increasing the energy associated with one remote station increases the interference associated with other nearby remote stations. As such, the radio communication system must strike a balance between the requirements of all remote stations sharing the same common channel. A steady state condition is reached when the SIR requirements for all remote stations within a given radio communication system are satisfied. Generally speaking, the balanced steady state may be achieved by transmitting to each remote station using power levels which are neither too high nor too low. Transmitting messages at unnecessarily high levels raises interference experienced at each remote receiver, and limits the number of signals which may be successfully communicated on the common channel (e.g. reduces system capacity).
This technique for controlling transmit power in radiocommunication systems is commonly referred to as a fast power control loop. The initial SIR target is established based upon a desired quality of service (QoS) for a particular connection or service type. For non-orthogonal channels, the actual SIR values experienced by a particular remote station or base station can be expressed as: ##EQU1##
The SIR is measured by the receiving party and is used for determining which power control command is sent to the transmitting party.
A slow power control loop can then be used to adjust the SIR target value on an ongoing basis. For example, the remote station can measure the quality of the signals received from the remote station using, for example, known bit error rate (BER) or frame error rate (FER) techniques. Based upon the received signal quality, which may fluctuate during the course of a connection between the base station and a remote station, the slow power control loop can adjust the SIR target that the fast power control loop uses to adjust the base station's transmitted power. Similar techniques can be used to control uplink transmit power.
As radiocommunication becomes more widely accepted, it will be desirable to provide various types of radiocommunication services to meet consumer demand. For example, support for facsimile, e-mail, video, internet access, etc. via radiocommunication systems is envisioned. Moreover, it is expected that users may wish to access different types of services at the same time. For example, a video conference between two users would involve both speech and video support. One technique for handling the different types of data communication involved in these situations is to provide a different radio bearer for each service. A radio bearer provides the capability for information transfer over the radio interface and is characterized by attributes such as information transfer rate (i.e., bit rate or throughput) and delay requirements, etc. A radio bearer carries either user data or control signalling. Typically a bearer is used for a specific service, e.g., speech. A radio bearer may span several physical channels or multiple radio bearers may share a physical channel depending on the bandwidth requirements of each radio bearer. In addition to one or more physical data channels (PDCHs), the user will be allocated a physical control channel (PCCH) on which overhead control information is carried to the user, e.g., bit rate information of the associated PDCHs, transmit power control bits and pilot symbols, at a constant bit rate, which can be used to make the SIR measurements used in the fast power control loop process. One potential relationship between radio bearers and physical channels is illustrated in FIG. 1B. Therein two radio bearers (RB1 and RB2) provide data blocks to multiplexor 2. The selected blocks are provided with forward error correction (FEC) coding 4 and are then interleaved 6 prior to being spread using the spreading code associated with PDCH1 at 8. Similar branches, not completely shown, can be provided for PDCH2 and the PCCH. Each of the resulting physical channels is then summed at block 11 and scrambled at block 10 prior to transmission.
However, the various services, and therefore radio bearers, may have different QoS requirements. Thus, it would be desirable to provide a slow power control loop for each radio bearer (or at least each PDCH) to enable these different QoS requirements to be accounted for during the power control process.